Enamel microstructure ofPakicetus(Mammalia: Archaeoceti)

The tooth enamel of the earliest cetacean,Pakicetus, is described and compared to enamel of a primitive artiodactyl and a variety of primitive ungulate families.Pakicetusenamel organization, which is considered primitive for Cetacea, consists of a combination of radial and decussating enamel types. Prism patterns include prisms with open (horseshoe-shaped) and closed (circular) boundaries.Pakicetusenamel is similar to that of many primitive ungulates, includingDiacodexis, the earliest artiodactyl, and Mesonychidae, an archaic ungulate family that often is considered close to the ancestry of Cetacea. This finding is consistent with the hypothesis, originally proposed on the basis of other aspects of morphology, that artiodactyls, cetaceans, and Mesonychidae are closely related.